Most bicycles have an “exo” gear system, exposed to the air that really catches a lot of grime and needs a lot of cleaning and maintenance if you ride a lot and take it seriously. The alternative is a non-derailler transmission, a geared hub. This would be applicable to a single-speed bicycle, or even a belt driven bicycle. Here’s a Yahoo Group that discusses gear hubs.
One of the important aspects about sustainable bicycle commuting is to solve common problems with existing safety bicycles. Maintenance of derailers and chain grime are uncivilized and inconvenient. Chain drives are not as durable as belt drives. And geared hubs are essentially rust and maintenance free. A low maintenance bicycle is a great starter cycle for beginning bicycle commuters that don’t need a fast ride and want to cycle in casual work wear.
The only other thing that would lead you to further bicycle convenience are solid tires. There was a time I got one or two flats a week, especially in wet weather. I got pretty good at always having spare tubes in my bags. However, this skill really is a barrier to bicycle commuting in general.